Re-use the default logger for performance metrics

This removes the need for configuring anything extra. To make scraping
easier each line is prefixed by "metrics:".

Fixes #64

CHANGELOG

+v3.6.6
+  - Re-use the default logger when logging metrics data
+
 v3.6.5
   - Test against ruby 2.3
   - Instrument GitLab Shell and log metrics data to a file

lib/gitlab_metrics.rb

 require 'logger'
 require_relative 'gitlab_config'
+require_relative 'gitlab_logger'
 
 module GitlabMetrics
   module System
@@ -25,7 +26,7 @@ module GitlabMetrics
   end
 
   def self.logger
-    @logger ||= Logger.new(GitlabConfig.new.metrics_log_file)
+    $logger
   end
 
   # Measures the execution time of a block.
@@ -48,7 +49,7 @@ module GitlabMetrics
     real_time = System.monotonic_time - start_real
     cpu_time = System.cpu_time - start_cpu
 
-    logger.debug { "name=#{name.inspect} wall_time=#{real_time.inspect} cpu_time=#{cpu_time.inspect}" }
+    logger.debug("metrics: name=#{name.inspect} wall_time=#{real_time} cpu_time=#{cpu_time}")
 
     retval
   end

spec/gitlab_metrics_spec.rb

@@ -2,22 +2,18 @@ require_relative 'spec_helper'
 require_relative '../lib/gitlab_metrics'
 
 describe GitlabMetrics do
-  describe '::measure' do
+  describe '.measure' do
     it 'returns the return value of the block' do
       val = described_class.measure('foo') { 10 }
 
       expect(val).to eq(10)
     end
 
-    it 'write in a file metrics data' do
-      result = nil
-      expect(described_class.logger).to receive(:debug) do |&b|
-        result = b.call
-      end
+    it 'writes the metrics data to a log file' do
+      expect(described_class.logger).to receive(:debug).
+        with(/metrics: name=\"foo\" wall_time=\d+ cpu_time=\d+/)
 
       described_class.measure('foo') { 10 }
-
-      expect(result).to match(/name=\"foo\" wall_time=\d+ cpu_time=\d+/)
     end
 
     it 'calls proper measure methods' do